Unpowered Electromagnets

It is very simple logic.It is similar to contact operation of coils.That is hold on (sealing)contact.The attraction takesplace at the time energisation.After deenergisation it hold plate and no air media between the two.The reluctace is less.once if you separate it,the air media is more.The reluctance is more.Due to this it is not attracted.It needs again energisation.

G.Swaminathan

It seems like you have the answer inside your question, only remains to add that the residual magnetism is part of the coil/bar system as a whole.

Here, in Wikipedia, more detailed description of the phenomenon (in the section "Electromagnets and permanent magnets"):

"...In some materials, some of the domains may realign themselves. In this case, part of the original magnetic field will persist even after power is removed, causing the core to behave as a permanent magnet. This phenomenon, called remnant magnetism, is due to the hysteresis of the material. Applying a decreasing AC current to the coil, removing the core and hitting it, or heating it above its Curie point will reorient the domains, causing the residual field to weaken or disappear..."

(- Internal links left for further reference).

If it's remnant magnetism, why doesn't it show up on the gaussmeter? The 3G we measured is insufficient to explain the attraction, and is still present after the attraction is gone.

Did you measure as it was stuck, or later when releases and demagnetised?

Both - there was no difference. All readings were down in the noise - 0.7 to 3.5 G.

Where do you place the gaussmeter?

Being that it was a NASA project, we tested it ad nauseum, so we took data at some 30-odd points around the magnet/plate combination, then around each piece individually. Before power was applied, during the application of power, and with no power. An eight hour exercise in a very small EMI shielded enclosure.

Interestingly, to me - because I've spent the past 30 years trying to forget my fields class - the peak flux on the powered magnet was 200G. The peak flux on the magnet/plate combo was 400G. I'm guessing the doubling was due to the tiny air gap between the magnet and the plate.

Request all the friends to type full URL instead of "Link" eg

instead of "description" type See <http://en.wikipedia.org/wiki/Electromagnet>

As browsers other than IExplorer do not copy the hidden link

How large is the contact area? In most cases, steel plates are oil coated and becomes sort of a binder when another plate or surface is pressed hard on it. This is true on very smooth surfaces, say glass, even water will hold them together if you pull them apart in opposite direction. Have you tried removing the magnet with a sliding motion?

No oil on the steel plate. This plate was specially machined as part of a NASA project. I can assure you it was quite clean. We were told specifically not to slide the plate when removing it, as that might have magnetized the plate if there was any remnant left on the electromagnet. So I pulled it straight up.

Both the end of the magnet and the plate are circles of about 2.5 inches diameter.

Steel acquires almost permanent magnetism. Confirm that steel is an alloy with its right composition. Then due to that its dipoles remain in one direction even when magnetism is withdrawn. i.e. there is dipole moment.

and this dipole moment which exerted several pounds of force would not register on a gauss meter?

I think I have the answer. When you magnetize the magnetic circuit plate + magnet you reach a high B value since the gap is small. If you look at the curve H-B of an Alnico Magnet (steel has a similar behavior you will notice that the ration B/H is very high. When you separate the parts the B-H relation depends on the air gap permeability and cuts the B-H curve at a low point. This is the remnant field. When again you near the plate the gap decreases and the B value goes up but NOT on the magnetization curve it goes up on reverse curve which has a much lower slope so that the B value when plate and magnet are together is a lot smaller than the one obtained during magnetization. Since the force is proportional to B^2 it will be very low as you noticed.

This behavior is the reason why systems using alnico magnets were magnetized after assembly. The new permanent magnets SaCo or similar have a very high coercive force and even if the gap increases they do come when it decreases to same values as before. The new magnets having this behaviour it is not any more needed to magnetize with the pole pieces they can be magnetized before assembly.

I am aware that without a sketch the explanation is not very easy to follow but if it will be necessary I shall make one I will not be able to do it this week but ask me if you need it next week.

No, I follow you. Thanks.

This might be sort of off the wall for some but, if ya plate and the surface of the magnet were to be very flat, static it self could very easily hold them together and it would need a bit of force to pull them apart.